1. Field of the Invention
This invention relates to a process and apparatus for the synthesis of ammonia. More particularly, this invention provides higher per-pass conversion of synthesis gas to ammonia, which results in lower recycle gas compression requirement, lower refrigeration requirement, and lower overall plant energy consumption, and, at the same time, substantially lower investment than known processes.
2. Description of the Prior Art
Ammonia is produced commercially by continuous processes which involve the seemingly straightforward reaction between stoichiometric amounts of nitrogen and hydrogen: EQU N.sub.2 +3H.sub.2 .fwdarw.2 NH.sub.3.
In practicing such processes, a gaseous mixture containing nitrogen and hydrogen is passed sequentially over one or more catalyst beds containing, for example, granular iron or promoted iron catalyst, at elevated pressure and temperature.
The reaction is accompanied by a reduction in gas volume, and equilibrium is therefore shifted to the right as the reaction pressure is increased. Commercial processes are known in which synthesis is carried out over a wide range of pressures, from about 20 to 1000 atmospheres, but most present-day commercial processes employ pressures in range of about 60 to 300 atm.
The reaction is exothermic; therefore, equilibrium is shifted to the right as the reaction temperature is lowered. However, at any given gas composition, the reaction rate velocity constant decreases as the temperature is lowered, so that as a practical matter, the temperature must be maintained at a high enough level to permit the synthesis of acceptable quantities of ammonia product in a reasonably short time. This is true even with acceleration of the reaction rate achieved with a catalyst.
For minimum catalyst volume, the temperature at each point in the catalyst would be controlled at the level at which the reactivity and the equilibrium driving force corresponding to the composition at that point are balanced to achieve the maximum rate of ammonia formation. In such an ideal system, both the temperature and the rate of heat removal would be highest at the inlet of the catalyst, with both gradually decreasing to lower levels at the outlet.
Older commercial processes attempted to approach these conditions for minimum catalyst volume by imbedding indirect heat transfer surfaces throughout the catalyst bed, by which heat could be transferred by indirect heat exchange to a cooling fluid such as incoming feed gas or other cooling media.
It was later discovered, however, especially for larger plants, that as a practical matter the costs of fabrication, maintenance, catalyst loading, and catalyst unloading of such systems were unnecessarily high, and that a more practical and more economical approach is to employ a series of two or more adiabatic beds with successively lower outlet temperatures. Most modern processes employ this approach.
In such processes, as the gaseous mixture passes through each bed, the ammonia concentration increases as hydrogen and nitrogen react. The temperature of the gas is also increased by the exothermic heat of reaction, until the ammonia concentration and temperature approach equilibrium conditions.
To achieve further conversion, the gaseous mixture is withdrawn from the first bed, cooled to a lower temperature at which the equilibrium concentration of ammonia is greater, and then introduced to the second bed, where the phenomena occurring in the first bed are repeated, except at higher ammonia concentration levels and lower outlet temperatures. In many processes, additional beds are employed in the same manner to obtain still greater ammonia concentrations.
Two general methods are used to cool the gas leaving a bed before sending it to another bed. One method is to quench directly the gas leaving a bed by mixing with it a part of the feed gas having a lower temperature, which results in a mixture having a lower temperature than that of the effluent before mixing. When two or more such direct quench steps are used, the cooler feed gas is divided into one part for each quench step and another part which is pre-heated and fed to the first bed. The other interbed cooling method is indirect heat exchange with another fluid.
The preceding comments on interbed cooling do not apply to cooling the effluent from the last bed in the series, which is always done by indirect heat exchange, even when direct quench is used to cool the gas between beds.
A prior art process which has found wide application in large modern plants is described in U.S. Pat. No. 3,851,046 to Wright et al. Two adiabatic beds are used, preferably in two separate reactor vessels, the effluent from the first bed being cooled by indirect heat exchange with feed gas to the first bed.
U.S. Pat. Nos. 4,744,966, and 4,867,959 to Grotz, the disclosures of which are specifically incorporated herein by reference, describe processes using two or more beds, preferably each in a separate reactor, the effluent from each being cooled by indirect heat exchange. The effluent from the first bed is cooled first by indirect heat exchange with the feed to the first bed as in the Wright, et al., patent, and then further cooled by indirect heat exchange with an external high temperature heat sink fluid, such as a steam generator or steam superheater.
Grotz '959 also recites a process in which the first of the two or more beds is a quench type converter. The first bed, preferably in a first separate reactor, is divided into two or more sub-beds, preferably in the same first reactor. The effluent from the first sub-bed is cooled by direct quench with part of the feed gas to the first bed.
Many efforts have been made to increase ammonia concentrations and associated per-pass conversion. Because the flow of circulating gas is roughly inversely proportional to the conversion per pass, many of the equipment and piping sizes in the synthesis loop and the energy required for recycle and refrigeration compression can be reduced roughly in the same proportion when conversion is increased.
Such efforts, however, prior to the present invention, have not resulted in totally satisfactory processes, as will be shown in further detail hereinafter. The need thus exists for a process for synthesizing ammonia at high reactor outlet ammonia concentrations at costs lower than those of known methods.